1. Technical Field
The present invention relates to an organic electroluminescent device (OELD), more particularly, to an OELD having reduced consumption power and improved ambient contrast ratio.
2. Related Art
An OELD of new flat panel display devices is a self-emitting type. The OELD has excellent characteristics of a view angel, a contrast ratio and so on. Also, since the OELD does not require a backlight assembly, the OELD has low weight and low power consumption. Moreover, the OELD has advantages of a high response rate, a low production cost and so on.
The OELD may be classified into a passive matrix type and an active matrix type.
In the passive matrix type OELD, anodes are arranged perpendicular to cathodes. The intersection of the cathode and anode make up the pixel where light is emitted. External circuitry applies current to selected strips of anode and cathode such that one pixel is turned on and other pixels are turned off. The brightness of each pixel is proportional to the amount of applied current. The passive matrix type OELD is easy to make, but more power is required.
In the active matrix type OELD, a thin film transistor (TFT) as a switching element is positioned in each pixel. A first electrode, which is connected to the TFT, is turned on and off in each pixel, and a second electrode facing the first electrode serves as a common electrode. In addition, an applied voltage is charged in a storage capacitor, the voltage is maintained by next frame. Accordingly, the active matrix type OELD requires a lower power, so it is efficient for large display devices.
FIG. 1 is a circuit diagram showing a pixel region of the related art OELD. As shown in FIG. 1, a gate line “GL”, a data line “DL”, a power supply line “PL”, a switching thin film transistor (TFT) “STr”, a storage capacitor “StgC”, a driving TFT “DTr”, and an organic electroluminescent diode “E” are formed in one pixel region “P”. The gate line “GL” and the data line “DL” cross each other such that the pixel region “P” is defined, and the power supply line “PL” is formed to be parallel to the data line “DL”. The switching TFT “STr” is fanned at crossing portion of the gate and data line “GL” and “DL”. The driving TFT “DTr” is electrically connected to the switching TFT “STr”.
The driving TFT “DTr” is electrically connected to the organic electroluminescent diode “E”. In more detail, a first electrode of the organic electroluminescent diode “E” is connected to a drain electrode of the driving TFT “DTr”, and a second electrode of the organic electroluminescent diode “E” is connected to the power supply line “PL”. The power supply line “PL” provides a source voltage to the organic electroluminescent diode “E”. The storage capacitor “Cst” is disposed between gate and source electrodes of the driving TFT “Tr”.
When a signal is applied to the switching TFT “STr” through the gate line “GL” such that the switching TFT “STr” is turned on, a signal from the data line “DL” is applied to the gate electrode of the driving TFT “DTr” such that the driving TFT “DTr” is turned on. As a result, light is emitted from the organic electroluminescent diode “E”. In this case, when the driving TFT “DTr” is turned on, a level of an electric current applied from the power supply line “PL” to the organic electroluminescent diode “E” is determined such that the organic electroluminescent diode “E” can produce a gray scale. The storage capacitor “StgC” serves as maintaining the voltage of the gate electrode of the driving TFT “DTr” when the switching TFT “STr” is turned off. Accordingly, even if the switching TFT “STr” is turned off, a level of an electric current applied from the power supply line “PL” to the organic electroluminescent diode “E” is maintained to next frame.
An organic emitting layer of the OELD may include organic emitting materials for emitting red, green and blue colors. Alternatively, the organic emitting layer may include an organic emitting material for emitting a white light, and red, green and blue color filter patterns are formed in each pixel.
Recently, to increase brightness and reduce power consumption, an OELD displaying a color image using a white pixel with red, green and blue pixels is introduced.
On the other hand, to improve an ambient contrast ration (ACR) of the display device, a circular polarizer is positioned at outmost side of the display device. Unfortunately, the circular polarizer causes an increase of power consumption.
To prevent the increase of power consumption, the color filter patterns are used for increasing the ambient contrast ratio. However, there is a limitation in increasing the ambient contrast ratio because there is no color filter pattern in the white pixel. In more detail, the OELD including a white organic emitting layer uses color filters to display a color image. In the OELD is intended to increase the ambient contrast ratio without the circular polarizer. However, since there is no color filter pattern in the white pixel, there is bad effect on the ambient contrast ratio. On the other hand, the OELD including the red, green and blue color filter patterns without the white pixel has an advantage in the ambient contrast ratio but has a disadvantage in power consumption.
Generally, in the OELD including red, green, blue and white pixels, power consumption is reduced when a color coordinate of a device displaying a white light is equal to a reference color coordinate of a panel. In general, since a color coordinate of a device displaying a white light is less than a reference color coordinate of a panel, the reference color coordinate becomes less by driving the blue pixel, which has less transmittance. In this case, since the blue pixel is also driven with the white pixel, power consumption is increased.